In wire electric discharge machining apparatuses, the machining conditions are adjusted such that the machining energy is increased as much as possible to improve the machining speed. However, when the machining energy is increased, the wire electrode becomes easy to break. Moreover, when a workpiece is machined into a shape that tends to make machining unstable, the wire becomes easy to break. In addition, for example, aging of the components of the wire traveling system causes the wire to travel unstably, which causes the wire to be easily broken. In other words, wire breakage is almost inevitable in wire electric discharge machining apparatuses.
If wire breakage is not detected fast and accurately and the machining power source and wire traveling are not stopped, electric discharge occurs at places other than at the workpiece, which is very dangerous. Consequently, a breakage detecting unit is indispensable for wire electric discharge machining apparatuses.
Methods have been used in which a limit switch, optical sensor, tension sensor or the like is provided on the wire traveling path as a breakage detecting unit. Moreover, a method is disclosed in Patent Literature 1 in which wire breakage is detected by using the rotational speed difference between the rotational components at two locations that are arranged on the wire traveling path. It is described that, with this method, breakage can be detected without causing a malfunction by detecting the rotational speed at any two of the wire supplying bobbin on which the wire electrode is wound, the tension roller that applies tension to the wire electrode paid out from the bobbin, the collecting roller that causes the wire electrode to travel at a predetermined speed, and the guide pulley that stabilizes the wire electrode as it travels and changes the traveling direction, and then, when the rotational speed difference between the two locations exceeds a predetermined threshold, determining that breakage has occurred.